Solder alloy, substrate with solder alloy for mounting electronic part, member to be bonded of electronic part, and electronic-part-mounted substrate

ABSTRACT

Disclosed is a solder alloy in use for bonding electric or electronic parts, containing: 3 to 12% by weight of a zin component; and a tin component. The oxygen content of the solder alloy is reduced to 100 ppm or less. Using the solder alloy, a bonding portion is formed on the substrate and the electronic part is mounted thereon to obtain a substrate for mounting the electronic part and a substrate on which the electronic part is mounted. The bonding portion made of the above solder alloy can prevent migration.

This application is a divisional of U.S. patent application Ser. No.09/619,445, filed Jul. 19, 2000, now U.S. Pat. No. 6,457,632 which is adivisional of U.S. patent application Ser. No. 08/936,118, filed Sep.24, 1997 now abandoned. Both applications are incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solder alloy useful for bonding ofelectronic parts, a substrate with the solder alloy for mountingelectronic parts, a bonding member of the substrate which is to bebonded to the electronic part, and an electronic-part-mounted substrate.More particularly, the present invention relates to technique to bond anelectronic part onto a substrate such as a circuit substrate and aprinted circuit board substrate with use of a bonding metalliccomposition containing no lead, a substrate for mounting electronicparts which is suitable for application of that bonding technique withno lead, an electronic-part-mounted substrate obtained thereby, and amember which is provided on the substrate to be bonded to the electronicpart.

2. Description of the Prior Art

Soldering is an art for bonding a substance by using a substance havinga low melting point, and has been used since old times. It is generallysaid that the origin of the soldering can be traced up to ancientMesopotamian civilization. In current industries, soldering is widelyused in bonding electronic devices, for example, bonding such electronicdevices as semiconductor, microprocessor, memory and resistor to asubstrate. Its advantage is not only to fix a part to the substrate, butalso to form electrical connection by electric conductivity of the metalcontained in the solder. This point is different from organic seriesadhesive agents.

The solder which is generally used is eutectic solder composed of tinand lead, having a eutectic point of 183° C. This is used for bondingsheet materials of aluminum or copper. It is characterized by theeutectic point which is not only lower than the melting point ofmetallic base material to be soldered, but also lower than a temperaturein which gasification of thermosetting resin begins. Further, it hasbeen known that tin component of the eutectic solder forms a particularintermetallic compound on an interface with a copper plate, therebyintensifying bonding strength between the solder and copper. In additionto the eutectic solder composed of tin and lead having such acharacteristic, solder composed of tin and zinc, solder composed ofsilver and tin, etc. have been used on trial. However, theirwettabilities are poor, thereby providing a poor connectability. Thusthey have not been used in actual fields.

As described above, bonding by soldering is still important inmanufacture of electronic devices. In today's world in which personalelectronic devices such as personal computer, mobile telephone and pagerhave been spreading quickly, the importance of solder in electronicdevice mounting technology has been intensified.

Spreading of electronic devices contributes to enrichment of people'slife. However, on the contrary, if a large amount of electronic devicesdisused are scrapped, there is a fear that wasted electronic devices maypollute the environment.

From such a condition, bonding skill by using solder containing no leadhas been demanded. However, solder in which lead is substituted by othermetal or solder containing a combination of other metals cannot behandled at such a low temperature that bad influence upon the base metalby high temperatures can be avoided, and the wettability is so poor thatthe solder is not fixed to the base metal satisfactorily. Thus, such asolder can be applied to neither fine soldering treatment such asmounting in semiconductor devices or ordinary bonding by solder.Particularly, solder with tin and zinc has too many problems to besolved, therefore, it is considered impossible to use it for actualapplication in electronic mounting.

To enable use of solder without lead in fine soldering works such asthick film formation, conductor circuit formation and semiconductormounting, a screen printing method using solder paste in which solderpowder and flux are mixed has been proposed. The flux used in solderpaste is generally classified to organic compound, inorganic compoundand resin. In the case when organic compound or resin is used, halogensalt, organic acid salt and the like of organic acid and amino group arefrequently added as active ingredient. In the case of inorganiccompound, ammonium halide, zinc halide, tin halide, phosphoric acid,hydroacid halide or the like are often added. Since these additivescorrode metals, inspection for corrosion due to flux residue afterreflow of solder paste is necessary. Moreover, organic substanceevaporating when paste is heated to remove flux, must be treated.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a solderalloy which contains metals applicable for wide fields instead of leadwhich may cause environmental pollution when a product using such asolder is wasted, and which is suitable for mounting electronic parts ona substrate.

Another object of the present invention is to provide a substrate formounting electronic parts with a bonding portion which is provided witha solder containing metals applicable for wide fields instead of lead.

Still another object of the present invention is to provide anelectronic-part-mounted substrate on which the electronic part ismounted using a solder containing metals applicable for wide fieldsinstead of lead.

Still another object of the present invention is to provide anelectronic-part-mounted substrate having a bonding portion in whichmigration is prevented.

An solder alloy in use for bonding electric or electronic partsaccording to the present invention comprises: approximately 3 to 12% byweight of a zin component; and a tin component, wherein the oxygencontent of the solder alloy is reduced to 100 ppm or less.

The solder alloy preferably comprises substantially no bithmus componentand no indium component.

The solder alloy is substantially a binary alloy essentialy consistingof the zinc component and the tin component such that the content ofother metallic component except for the zinc component and the tincomponent is reduced to less than 0.1% by weight.

A substantially binary solder alloy in use for bonding electric orelectronic parts according to the present invention essentially consistsof: approximately 3 to 12% by weight of a zin component; a balance tincomponent; and an oxygen component at a content which is reduced to 100ppm or less.

Moreover, a substrate for mounting an electronic part of the presentinvention comprises: an electrically conductive bonding member providedon the substrate; and a bonding portion which is formed of a solderalloy on the electrically conductive bonding member, in which the solderalloy contains a zinc component and a tin component and has a firstlayer at the interface of the electrically conductive bonding member andthe bonding portion, and the content of the zinc component of the firstlayer is concentrated to approximately 70% by weight or more.

In the substrate, the first layer has a thickness of approximately 1 μm.

The contentn of the zinc component of the first layer is 88% by weightor more.

The solder alloy has a second layer being adjacent to the first layer,and the content of the zinc component of the second layer is 5.8 to11.7% by weight.

Another substrate on which an electronic part is mounted, according tothe present invention, comprises: an electrically conductive bondingmember provided on the substrate; and a bonding portion connecting theelectronic part with the electrically conductive bonding member of thesubstrate, the bonding portion being formed of a solder alloy on theelectrically conductive bonding member, in which the solder alloycontains a zinc component and a tin component and has a first layer atthe interface of the electrically conductive bonding member and thebonding portion, and the content of the zinc component of the firstlayer is concentrated to approximately 70% by weight or more.

Still another substrate for mounting an electronic part, according tothe present invention, comprises: an electrically conductive bondingmember provided on the substrate; and a bonding portion which is formedof a metallic composition on the electrically conductive bonding member,in which the metallic composition contains a zinc component and a tincomponent and includes a surface layer in which the zinc component isconcentrated so that the content of the tin component is at mostapproximately 1% by weight.

In the substrate, the surface layer has a thickness of approximately 30to 120 Å.

In the substrate, the content of the zinc component of the solder alloyis 0.5% by weight.

Moreover, a substrate on which an electronic part is mounted, accordingto the present invention, comprises: an electrically conductive bondingmember provided on the substrate; and a bonding portion connecting theelectronic part with the electrically conductive bonding member of thesubstrate, the bonding portion being formed of a metallic composition onthe electrically conductive bonding member, in which the metalliccomposition contains a zinc component and a tin component and includes asurface layer in which the zinc component is concentrated so that thecontent of the tin component is at most approximately 1% by weight.

Another substrate for mounting an electronic part, according to thepresent invention comprises: an electrically conductive bonding memberprovided on the substrate; and a bonding portion which is formed of asolder alloy on the electrically conductive bonding member, wherein theelectrically conductive bonding member has a top face and a concave sideface such that appeared as a curved line in a cross section of theelectrically conductive bonding member perpendicular to the substrate

In the substrate, the concave side face is of a saddle shape or ahyperbola shape.

In the substrate, the curved line in the cross section is curved at aradius of less than 100 mm.

In the substrate, wherein the side face includes two in continuous facesto form an angular portion therebetween.

In the substrate, the top face is concaved.

Moreover, a method of bonding an electronic part to a substrate with ametallic composition, according to the present invention comprises:disposing a metallic composition between the electronic part and thesubstrate, the metallic composition comprising 6 to 12% by weight of azinc component and a tin component; melting at least surface of themetallic composition to wet the electronic part and the substrate withthe molted metallic composition; and hardening the melted metalliccomposition, thereby bonding the electronic part to the substrate.

Another method of bonding an electronic part to a substrate with ametallic composition, according to the present invention, comprises:disposing a metallic composition between the electronic part and thesubstrate, the metallic composition comprising at least 0.5% by weightof a zinc component and a tin component; melting at least surface of themetallic composition to wet the electronic part and the substrate withthe molted metallic composition; and hardening the melted metalliccomposition, thereby bonding the electronic part to the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view for explanation of a structure of thesubstrate with a bonding portion for mounting an electronic partaccording to the present invention, and it includes a diagram forexplanation of a surface of the bonding portion of the substrate, and itincludes a diagram for explanation of the vicinity of a bondinginterface of the bonding portion;

FIG. 2A is a schematic structure diagram of an embodiment of thesubstrate for mounting an electronic part according of the presentinvention, and FIG. 2B is a schematic structure diagram of theelectronic-part-mounted substrate, on which the electronic part ismounted;

FIG. 3A is a schematic structure diagram of other embodiment of thesubstrate for mounting the electronic part according to the presentinvention, and FIG. 3(b) is a schematic structure diagram of anembodiment of the electronic-part-mounting substrate on which theelectronic part is mounted;

FIGS. 4A and 4B show vertical sectional views for explanation of anembodiment (FIG. 4A) and other embodiment (FIG. 4B) in which the bondingportion is formed on a bonding member of the substrate for mounting theelectronic part, using a bonding material, respectively;

FIGS. 5A and 5B show vertical sectional views for explanation of anembodiment (FIG. 5A) and other embodiment (FIG. 5B) in which the bondingportion is formed using a bonding material, on an example of the bondingmember of the substrate for mounting the electronic part according tothe present invention;

FIGS. 6A and 6B show vertical sectional views for explanation of anembodiment (FIG. 6A) and other embodiment (FIG. 6B) in which bondingportion is formed using a bonding material, on another example of thebonding member of the substrate for mounting the electronic partaccording to the present invention;

FIGS. 7A and 7B show vertical sectional views for explanation of anembodiment (FIG. 7A) and other embodiment (FIG. 7B) in which the bondingportion is formed using another bonding material, on the bonding memberin FIGS. 6A and 6B;

FIGS. 8A and 8B show vertical sectional views for explanation of anembodiment (FIG. 8A) and other embodiment (FIG. 8B) in which bondingportion is formed using a bonding material, on still another example ofthe bonding member of the substrate for mounting the electronic partaccording to the present invention;

FIGS. 9A and 9B show vertical sectional views for explanation of anembodiment (FIG. 9A) and other embodiment (FIG. 9B) in which the bondingportion is formed using a bonding material, on a further example of thebonding member of the substrate for mounting the electronic partaccording to the present invention;

FIGS. 10A and 10B show vertical sectional views for explanation of anembodiment (FIG. 10A) and other embodiment (FIG. 10B) in which thebonding portion is formed using a bonding material, on a still furtherexample of the bonding member of the substrate for mounting theelectronic part according to the present invention;

FIG. 11 is a schematic structure diagram showing an embodiment of anapparatus for supplying the bonding material according to the presentinvention;

FIG. 12 is a schematic structure diagram showing another embodiment ofthe apparatus for supplying the bonding material according to thepresent invention;.

FIG. 13 is a schematic structure diagram showing still anotherembodiment of the apparatus for supplying the bonding material accordingto the present invention; and

FIG. 14 is a chart of linear analysis of the amount of zinc, by energydiffusion type X-ray spectral analysis, in the vicinity of a bondinginterface between the bonding portion of the bonding material and abonding member of the substrate for mounting the electronic partaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Characteristics required for solder alloy are (1) excellent wettability,(2) ability of soldering at a temperature not thermally damaging adevice to be soldered, that is, melting temperature is in the vicinityof 473 K, (3) fragile intermetallic compound or embrittlement layer isnot formed by reaction with base metal, (4) ability of being supplied ina form suitable for automatization (solder paste, solder powder, etc.),(5) oxides of metallic compounds contained in solder alloy do not causedefects such as improper wetting, void, bridge and the like.Particularly, in bonding of electronic parts, since soldering isperformed in fine parts, to bond an electronic part to a base metal, thesurface tension, viscosity, flowability and the like of the solder alloyare important. Although conventional tin-lead solder has been often usedbecause the aforementioned requirements are satisfied, a substitutivematerial has been demanded in terms of influences upon the environment.

The solder according to the present invention is an alloy containing tinand about 3-12 wt % zinc, but not lead. Zinc and tin do not affect theenvironment badly because it has a high safety, and further tin/zincalloy can satisfy the aforementioned requirements for the solder.

As shown in FIG. 1, tin and zinc are solved in total range, so that,regardless of mixing ratio therebetween, an alloy is formed withoutproducing any deposit. Thus, they are excellent in continuity assuredwhen they are used as a solder. Particularly, tin-zinc alloy containing3 to 12 wt % zinc has a melting temperature of less than 493 K (220°C.), so that it is suitable for soldering. The composition which is mostsuitable for soldering is tin-zinc alloy containing about 9 wt % zinc,having the lowest melting temperature (liquidus temperature) of 471 K(198° C.). Even if the ratio of zinc is less than 3 wt % or more than 12wt %, the melting temperature is high. Generally, it is said that if thesoldering temperature for electronic parts is lower by 10° C., theservice life of the electronic part becomes double. Thus,.the fact thatthe melting temperature of the solder alloy is low is very effective.

Alloy formed in entire composition range by zinc and tin is solutionalloy of tin having an excellent electric conductivity, and itscharacteristics such as electric conductivity are almost the same assimple tin. Its wettability to copper, which is ordinarily used asconductor in PC board and the like, is also high. Since zinc is cheaperthan other metals, tin-zinc alloy can be supplied as a solder alloywhich is used in large quantity for electronic devices, at about thesame cheap price as the tin-lead solder alloy.

However, it has been conventionally recognized that tin-zinc alloy doesnot have sufficient wettability such that it can be used as a solder.Therefore, bithmus or silver has been mixed with the tin-zinc alloy touse as a solder. The present invention is based on a discovery that, ifthe oxygen content of the tin-zinc alloy solder is determined to be 100ppm or less, the tin-zinc binary solder suitably wets a base of metalsuch as copper and the like. The reason is that it is oxygen which makesthe solder alloy fragile and which reduces the wettability remarkably.Even if foreign matters such as nitrogen and hydrogen are contained inunavoidable quantity, there is no problem.

Preparatlon of tin-zinc ahoy solder having a small amount of oxygencontent can be achieved by using phosphor, magnesium or the like whichhave a low melting point and are likely to react with oxygen as oxygenscavenger or deoxidizer. If explaining in details, a raw zinc materialis added to a raw tin material depending on the composition ration of anaimed solder alloy and melted by heating. Then phosphor of 0.01-0.1% byweight ratio is added to the melted material. As a result, oxygen in themelted material is bonded with phosphor so that it floats on the surfaceof the melted material as slag. Then this slag is removed and the meltedmaterial is poured into a mold and left as it hardens. As a result,solid solder in which the oxygen content is reduced to 30 ppm or less isobtained.

In common solder materials, it is permissible to add one or acombination of two or more components selected from antimony, indium,gold, silver and copper. Adding antimony or copper can further improvethe wettability of the solder alloy. Adding indium, gold or silver canreduce the melting temperature of the solder alloy. If theaforementioned additive is applied to the tin-zinc alloy, the sameeffects can be expected. However, if the amount of these elementsexceeds 3 wt %, solder gloss is vanished so that the appearance is poor.And, if such an additive component is used, it is desired to be addedafter the amount of oxygen in a solution of tin and zinc is reduced byusing oxygen scavenger. On the other hand, binary alloy solder composedof substantially tin and zinc without using such additive component isexcellent in that there is no fear in changes of solder characteristicsdue to generation of ternary eutectic substance or intermetalliccompound after soldering. In particular, if bithmus or indium is used asan additive, it becomes difficult to recover usably purified materialsfrom the solder alloy for recycling use. Moreover, the alloy solder oftin and zinc containing no bithmuis and no indium is quite preferablefor bonding of electronics devices by the reason which is describedbelow.

Since zinc is a compound easy to be oxidized, the characteristic of thetin-zinc alloy solder is likely to be damaged by generation of oxides.However, this problem can be solved by soldering in substantiallynon-oxidizing environment such as nitrogen and argon gas, so thatwetting fault and bonding fault due to oxidization can be prevented.

If oscillatory wave energy is supplied to a surface to be soldered byultrasonic wave or the like at the time of soldering, solderingwettability is improved.

Tin-zinc alloy solder according to the present invention can be usedeffectively for soldering of various electronic devices and installationand assembly of semiconductor devices. Of course, it can be used forapplication field in which conventionally tin-lead solder has been used.

(Surface Zinc Layer)

As integration of semiconductor devices such as circuit substrate and ICchips is increased, a bonding member for electrically connecting thesemiconductor devices together or a-semiconductor device with otherparts, i.e. pads, wiring parts, etc., has become fine. Thus, the bondingby soldering for bonding these parts also has become fine. If such finebonding is formed by the conventional tin-lead solder, generation ofmigration is currently a problem.

According to the present invention, it has been found that a bondingformed by soldering of a substrate with a solder containing tin and zinccan prevent generation of migration. As a result of research on thisreason, it has been found that the melted solder containing tin and zincis hardened so as to produce an outer peripheral portion or a surfacelayer in which the zind component is highly concentrated so that doesscarcely contain tin (1 wt % or less detected by Auger ElectronSpectroscopy), which will be called “a surface zinc layer” hereinafter,thereby preventing a migration.

The surface zinc layer, which is formed on the hardened material of themelted solder containing tin and zinc can be classified into twocategories. One is (1) surface zinc layer which is formed on the surfaceof a hardened substance (particularly a portion adjacent to atmosphere)having a thickness of about 40 to 120 Å, containing a little tin (atmost 1 wt %). Another is (2) surface zinc layer which is formed on aninterface with a metallic bonding part, having a thickness of 0.5 to 2μm. The surface zinc layer of (1) is ordinarily composed of zinccontaining oxygen (or oxidized zinc). If oxygen is remarkably removedfrom the atmosphere in which the soldering is carried out, the surfacezinc layer of (1) may be divided into an outer layer located on the mostsurgical portion, composed of zinc containing oxygen, and an inner layerlocated insider thereof containing little oxygen. The surface zinc layerof (2) contains a small amount of oxygen. In the surface zinc layer of(2), nearer the surface, the amount of tin decreases and a ratio of zincincreases. The most surface portion of the layer (2) contains littletin. The zinc content on average of the surface zinc layer of (2) variesdepending on composition of melting solder, and it is generally at leastabout 70 wt %.

It is considered that the surface zinc layers of (1) and (2) aredifferent in formation process. The surface zinc layer of (1) seems toshow such an inclination that zinc is formed in thin layer on thesurface of the melted material due to a difference in physical propertysuch as surface tension or the like, and this thin layer is hardened soas to produce a surface zinc layer. The surface zinc layer of (2) seemsto show such a inclination that, if the melting solder makes contactwith a metal (or a specific substance like copper, etc.), zinc movestoward that metal and it is then concentrated in the vicinity of thebonding interface so as to form a layer of (2). Because zinc is a metallikely to be oxidized, zinc existing on the surface of the meltingsubstance absorbs oxygen in the environment, so that the layercontaining oxygen like the aforementioned (1) is formed so as to inhibitpenetration of oxygen inside thereof.

Thus, if metallic composition containing tin and zinc goes through aprocess of melting and then hardening, the hardened material is sostructured to have a surface zinc layer. Accordingly, if a bondingportion is formed by using a melted tin-zinc compositioncorrespondingly, it has little tin on its surface, thereby preventingmigration. Thus this material is suitable for bonding of high densitymounting substrates.

If actually substrate pad is soldered with tin-zinc eutectic solder, thecondition shown in FIG. 1 is obtained. FIG. 1 includes a sectional viewof a soldered substrate, in which, if a surface portion 1 of solder Scovering the pad P of the substrate B is analyzed in the depth directionaccording to Auger electron spectroscopy method, a structure in thevicinity of the surface is also shown in FIG.1. According to thisexpression, zinc oxide layer 3 having a thickness of about 30 to 120 Åexists on the most external surface and below this, a layer 5 in whichtin and zinc are mixed exists. On the other hand, if a portion 2 in thevicinity of a bonding interface between the solder S and the pad P isanalyzed, as shown in FIG. 1, zinc layer 6 containing a little amount ofoxygen (1-30 ppm) and having a thickness of about 0.5 to 2 μm exists onthe interface, and above this, a layer 7 in which tin and zinc are mixedexists. The layer 5 is continuous with the layer 7 in FIG. 1. Although aratio of content of zinc in the layer 6 is more than 70 wt % at averageof the entire layer, the most surface portion contains little tin.

As understood from the above description, if a bonding portion is formedin accordance with a method using a process of melting-hardening by usea metallic composition containing tin and zinc, a surface zinc layercorresponding to the above (1) is formed. If bithmus or indium iscontained in that metallic composition, it inhibits the surface zinclayer of (1) from being formed on the hardened composition. Thus, thebonding portion according to the present invention is preferred to beformed with binary composition of tin/zinc, although it is notrestricted to the binary composition and may contains other componentsexcept for bithmus and indium. Moreover, because there is no fear inchanges of physical property due to generation of ternary eutecticsubstance after mounting if looking from viewpoints of bonding of thesubstrate, the binary composition of tin-zinc is favorable. Particularlybecause of good economic performance ensured by low melting point,tin-zinc eutectic composition (zinc =about 9 wt %) is desirable.

When the melted metallic composition contains zinc, an obtained hardenedmaterial contains surface zinc layer. If the amount of contained zinc issmall, it is considered that formation of the surface zinc layer isinsufficient. Table 1 shows a result of research on a relation betweenthe ratio of contained zinc and generation of migration when substrateare bonded by using metallic composition in which the amount ofcontained zinc differs. In this table, a case when a bonding havingmigration is 0 is shown by a letter A, a case when the bonding havingmigration is less than 20% is shown by a letter B and a case when thebonding having migration is more than 20% is shown by a letter C.

TABLE 1 Ratio of zinc content (wt %) 0 0.01 0.1 0.5 Generation ofmigration C B B A

From Table 1, it has been found that the tin-zinc solder can preventgeneration of migration in fine bonding portions when the solder inwhich lead is mixed at a ratio of 0.01 wt % or more, preferably at theratio of 0.5 wt % ore more. Thus, in a bonding portion having thesurface zinc layer on its surface, migration in a high density mountingbonding such as bonding of a bump or lead which is arranged with aninterval of 1 mm or less is prevented effectively.

As is evident from the above description, the bonding portion achievedby tin and zinc through a process of melting and hardening gets astructure of having the surface zinc layer, following a mechanism inwhich the above zinc layer (1) is formed. Since migration is preventedby formation of zinc layer on the surface and then substantialnonexistence of tin on the surface, the bonding portion formation methodshould not be restricted to soldering, but the composition containingtin and zinc forms a bonding portion through the process ofmelting—hardening. Of course, it is permissible that the entire bondingcomposition is not melted, but only the surface portion is melted. Or astructure in which zinc layer is formed on the bonding portion,regardless of mechanism in which the zinc layer is formed is alsopermissible. Thus, a formation method for the bonding portion may beselected appropriately depending on requirement, from for example,welding methods including arc welding, electronic beam welding, plasmaarc welding, laser welding and optical beam welding; press-weldingmethods including resistance welding, cold welding, friction welding anddiffusion welding; hard brazing methods including resistance brazing andvacuum brazing; and soldering methods including laser brazing andimmersion brazing. Or it is permissible to use a solder supplying unit,in which solder particles are transported to a portion to be bonded byusing inactive gas as carrier, and in which the solder particlestransported are heated in such a condition that the surface of thesolder particles is being melted, when they arrive at the material to bebonded, so as to wet the materials to be bonded with the solder.

A bonding portion having the surface zinc layer can be formed on varioussubstrates. For example, paper substrate copper stretching integratedboard such as a paper-phenol substrate copper stretched integrated boardand paper-epoxy substrate copper stretched integrated board; a glasssubstrate copper stretched integrated board such as glass clothing epoxymulti-layer wiring board and glass clothing polyimide multi-layer wiringboard; composite copper stretched integrated board such as epoxycomposite copper stretched integrated board, ceramic substrate such asflexible wiring board, multi-wire board, thick film circuit substrateand thin film circuit substrate; multi-layer wiring substrate in whichvarious materials are combined; composite substrate such as enameledsubstrate and metallic base substrate; and silicone substrate which is asemiconductor material are provided.

If classified in terms of substrate mounting style, single surfacemounting, double surface mounting, double surface mounting equipped withlead attached parts, single surface mounting equipped with lead attachedparts, lead through mounting, and the like are provided. The mountingpart include ceramic, capacitor, inductor, Jumper, transistor, diode,aluminum electrolytic capacitor, tantalum semi-fixed resistor, trimmer,coil and the like, as a passive part. As an active part, it includes ICand SI, which are typical parts. If classified depending on externalshape and configuration of the package, SOIC, SOP, QIP, QFP, PLCC, LCC,SOJ, MSP, BGA, FC-BGA, CSP, PLC, MCM, OE-MCM, and high density mountingdevice in which chips are stacked are available.

Further, this is applicable to not only field of mounting substrate, butalso other fields. For example, that field includes bonding of ICpackage and CPU for use in semiconductor field, bonding of electriccircuit in a hard disk or LCD panel contained in personal computer,cable connector often used in connection of personal computer andprinter, optical connector often used in transmission cable, bonding ofa radiator of vehicle and the like.

FIG. 2A shows an example of a substrate in which a bonding portionhaving a surface zinc layer is formed on the pad. On an electricallyconductive pad 12 provided on the substrate 11 made of resin is formed abonding portion 13 by tin-zinc solder. For example, as shown in FIG. 2B,on one side of the substrate 11, such parts as resistors, QFP and thelike are connected with the pad 12 through the bonding portions 13. Onthe other side thereof, a part such as a display unit is connected withthe pad 12.

FIG. 3A shows an example of a multi-layer substrate in which a bondingportion having a surface zinc layer is formed on the pad. On the pad 18provided on the surface of the multi-layer substrate 17 made of resin,containing a copper conductive layer 16 inside thereof is formed abonding portion 19 composed of tin-zinc solder, and then, for example asshown in FIG. 3B, a part 20 is connected to the pad 18 through thebonding portions 19.

(Zinc Layer on Bonding Interface)

If an amount of oxygen is small, that is, the content of oxygen insolder is less than 100 ppm, the wettability of zinc is not so inferiorto that of lead. Therefore, the tin-zinc solder indicates a sufficientwettability and the layer composed of mainly zinc such as the surfacezinc layer of (2) mentioned above acts like adhesive agent so that itbonds a member to be bonded favorably. Particularly if the amount ofzinc in the layer is about 88-99.5 wt %, a bonding having an excellenttensile strength is formed. This is very effective for bonding a memberto be bonded such that the surface is composed of a metal like copper,silver, nickel, gold, palladium, platinum and iridium. Particularly thewettability to copper, silver and gold is excellent.

In other words, if a layer composed of mainly zinc having a small amountof oxygen like the surface zinc layer of (2) is made to intervene,soldering of metallic parts can be carried out excellently. Thus, it isnot necessary to restrict the layer composed of mainly zinc to a layercaused from melted solder. Thus, the layer composed of mainly zinc canbe formed not only by various welding methods, pressure welding methodsand brazing methods which are described above in formation of thebonding portions, but also may be formed directly on a member to bebonded through a plating method or the like. In a case when the memberto be bonded is composed of a material to which the wettability of zincis not so good, the surface of the member to be bonded is covered withcopper, silver, gold or the like, before a layer composed of mainly zincis formed.

If bonding is formed through melting in such a condition that a layercomposed of mainly zinc exists between a member to be bonded andtin-zinc alloy, a bonding portion having an excellent tensile strengthis formed by tin-zinc alloy on the member to be bonded through a layercomposed of mainly zinc. For formation of this bonding, various methodsincluding welding method, pressure welding and brazing method describedpreviously are available. According to this method, tin-zinc alloy isformed on a member (wiring, pad, etc.) to be bonded of varioussubstrates, mentioned in a description of the surface zinc layer,through a layer composed of mainly zinc. This method is applicable forvarious mounting styles and can correspond to high density mounting. Orthis may be used for bonding and assembly of the aforementionedelectric/electronic parts and mechanical parts.

When tin-zinc alloy solder is melted and a substrate is dipped therein,bonding is achieved by placing the solder on the bonding portion of thesubstrate. In this case, as described above, solder is bonded in such amanner that a layer composed of mainly zinc of the above (2) is disposedon an interface of the member to be bonded, and the surface has asurface zinc layer of the above (1). If the solder which is in contactwith the member to be bonded is in melting state for a long time, somecomponent in the member to be bonded are gradually diffused so as topenetrate into the layer existing on the bonding interface and beingcomposed of mainly zinc. At the same time, zinc penetrates into thesurface of the member to be bonded. Thus, if the member to be bonded isin contact with the melted solder for a long time after bonding, thelayer composed of mainly zinc of the above (2) is likely to change inits composition.

The substrate having the above-described bonding portion on a member tobe bonded is advantageous in preventing the resist portion of thesubstrate from being damaged.

(Shape of Member to be Bonded)

When a bonding portion is formed on a member to be bonded such as a pador wiring by using the aforementioned metallic composition as solder,elements to determine the strength of the bonding include a shape of themember to be bonded. In the case of a pad 21 in which the cross sectionis trapezoidal like in FIG. 4A and FIG. 4B, the solder S applied is incontact with not only a top face 22 of the pad 21 but also inclined sidefaces 23, so that the area of the bonding interface is increased,thereby securing a bonding strength between the bonding portion and themember to be bonded. Regarding this point, as a shape furtheradvantageous of the member to be bonded, a shape shown in FIGS. 5A and5B can be presented. In FIGS. 5A and 5B, the side faces 25 of the pads24, inclined relative to the substrates, draw curved lines in sectionalview, and each of the curved lines is concave, that is, of a saddletype, hyperbola type and the like. In this case, the area of the bondinginterface is larger than the pad 21, so that a sufficient bondingstrength is easier to obtain. Further, because the side face thereof isconcave, solder adhering thereto is settled favorably, which iseffective in enhancing the wettability of solder. Still further, becausethe side face has concavities, a resistance to distortion along thesubstrate caused in the bonding portion due to a change in dimension ofthe substrate accompanied by a change in temperature is increased.

Referring to FIGS. 4A, 4B, 5A and 5B, each of the drawings of Aindicates a case in which the amount of solder is small and that of Bindicates a case in which the amount of solder is large. By forming theside face in a concave shape, an amount of solder which can be supportedby the side face is increased, so that it is possible to mount solder onthe pad stably. Consequently, adaptability when the amount of solder tobe applied changes is also increased.

FIGS. 6A and 6B show an example in which the concavities in the sidefaces of the pads are different. FIG. 6A shows a case in which theamount of solder is small. FIG. 6B shows a case in which the amount ofsolder is large. Since the concavity of the side face 27 of the pad 26in FIGS. 6A and 6B is milder than that in FIGS. 5A and 5B, the area ofthe bonding interface is smaller than in the case of the pad 24, so thatthe amount of solder to be mounted on the side face 27 is smaller. Anadvantage in forming the inclined side face of the pad in concave shapebecomes conceivable if the radius of curvature of the side face is lessthan 100 mm. It is desirable that there are many places or portionswhich satisfy this requirement on the side faces.

FIGS. 7A and 7B show an example in which the wettability of solder islow. Generally, if the wettability of solder is low, the wetting angleis increased so that the stability of solder on the pad side faces orholding performance is likely to drop. However, if the side face isconcave, as shown in FIG. 7, the holding performance of the solder S′ isnot reduced so much, so that a sufficient performance for correspondenceis secured in the case when the amount of solder is small as well as inthe case when the amount of solder is large. This can be understoodeasily if considering that solder having a low wettability is used incase of the pad 21 shown in FIGS. 4A and 4B.

Although FIGS. 5-7 show cases in which the inclined side faces of thepads are continuously concave, it is possible to obtain a similar effectin pads in which concave portions are formed by in continuous faces asshown in FIGS. 8A, 8B, 9A and 9B. A pad 28 in FIGS. 8A and 8B isprovided with a step portion 30 at a side face 29. A side portion 32 ofa pad 31 shown in FIGS. 9A and 9B is formed of two incontinuous faces soas to form an angular portion 33 thereby providing a concavity. In anypads, the area of the bonding interface is increased by the step portion30 and the angular portion 33, so that the holding performance of solderS is improved. A sufficient performance for correspondence is assured inthe case of 8A or 9A when the amount of solder is small and in the case8B or 0B when the amount of solder is large.

FIGS. 10A and 10B show an example in which the area of the interface isfurther increased. In this case, concave faces are formed in not onlythe side face 35 of the pad 34 but also the top face 34 thereof. Theside face 35 is concave like in the case of FIGS. 5 to 7, and the topface 34 is also inclined toward the center. These structures cancorrespond to the case when the amount of solder is small and the casewhen the amount of solder is large, like the cases of FIGS. 5 to 9.

The above FIGS. 5 to 10 show vertical sectional views. The entire padconfigurations are so structured as shown in this vertical sectionalviews. Thus, it can be easily understood that the above configurationscan be applied to substantially truncated cone or substantiallytruncated polygon pyramid pads for point bonding, having top and bottomcircles or polygons, and substantially frustum wiring or the like whichextends in a longitudinal direction and has a trapezoidal cross section,so that the cross sections are as shown in FIGS. 5-10.

(Solder Supplying Means having Oxide Film)

A means for supplying the aforementioned metallic composition containingtin and zinc to a member to be bonded, that is, an embodiment of a meansfor soldering may be a means which supplies melted material onto themember to be bonded through fine openings like a nozzle and a screen. Insuch a means, shutting out of the melted material in the opening is veryimportant so as to form fine bonding portions. As regards this point, byforming a metallic oxide film on the surface of the opening forsupplying the melted material, the shut-out of the melted material isimproved. This utilizes such a characteristic that the surface of themetallic oxide solid can reject melted metal easily. As a result, thischaracteristic prevents the melted metal from stagnating on the metallicoxide film or adhering to the wall face. By covering the opening end andscreen slit of the nozzle with metallic oxide film, shut-out of themelted metal at the film position is improved. As a result, narrowing orclogging of the nozzle opening and slits does not occur, so that verynarrow nozzles can be used. Therefore yield rate is improved so thatcleaning work for the nozzle and slit becomes unnecessary. Further, ifthe shut-out of the melted metal is improved, the supply thereof can becontrolled strictly, so that the melted metal can be supplied accuratelyto fine portions. Concretely speaking, in the case of soldering finewiring or pads having an interval of less than 100 μm, it is possible toprevent formation of bridges.

The metallic oxide for covering the opening is not restricted, and maybe oxide of metallic element contained in the melted metal. However, inthe nozzle or screen for use as a means for supplying tin-zinc solder,metallic oxide such as spinel type chromate oxide is preferable, becauseit can be formed easily on the surface of stainless steel and thus itcan be used easily and favorable in the apparatus configuration. Thespinel type chromate oxide can be formed in the thickness of 1 μm on thesurface of stainless steel by chrome contained in stainless steel byheating the stainless steel at more than 1300° C.

As a means for discharging the melted metal from the nozzle, a type ofusing vibration energy and thermal energy as well as a pump for ordinaryuse can be used. According to the type of the vibration energy, byapplying vibration energy such as ultrasonic wave to the nozzle, themelted metal is discharged from the nozzle tip in the form of droplet.According to the type of the thermal energy, by applying heat to thenozzle cyclically, the surface tension of the melted metal in the nozzleis partially changed so as to discharge the melted metal in the form ofdroplet. When the melted metal is discharged in the form of droplet,covering the opening at the nozzle tip with metallic oxide is veryeffective. Particularly, if the nozzle opening is formed in taperedform, shut-out of the melted metal is further improved. As for thescreen, if the screen is formed such that the slit is slightly openforward, an operation for removing the screen is facilitated.

(Nozzle Unit)

FIG. 11 shows an example of the nozzle type soldering apparatusaccording to the present invention. This soldering apparatus 100comprises a melting bath 101 for heating and melting the solder, anozzle 103 connected to the melting bath 101, an oscillating unit 105attached to the nozzle 103, a heating means 107, a gas supplying unit109 for supplying inactive gas and a transfer unit 111. On an insidewall of the nozzle 103 formed is an metallic oxide film 129 of chromiumoxide. The solder is thrown into the melting bath and melted by heating.After that, the melted solder is supplied to the nozzle 103 at aconstant speed. Elastic wave oscillated from the oscillating unit 105 istransmitted to the melted solder S in the nozzle 103. Consequently, thesolder S is made to droplet S″ and regularly discharged from the nozzleport 103′ of the nozzle 103. The droplet S″ discharged from the nozzle103 reaches the substrate 115 which is a base metal placed on thetransfer unit 111. The nozzle 103 is provided with a hood 117 forpreventing oxidation of the droplet S by inactive gas supplied from thegas supplying unit 109. A controller 113 has a positioning controlfunction for controlling the transfer speed of the transfer unit 111 sothat the droplet S″ reaches an appropriate position on the substrate115. The transfer unit 111 has an oscillator for supplying elastic waveto the substrate 115. The wave length of the elastic wave transmitted bythe oscillator 125 and the oscillating unit 105 is controlled by afrequency adjusting unit 127.

In the above apparatus, the oscillating unit 105 is used as a means fordischarging solder, and it may be substituted by known means with sparkor bubbles used in ink jet technology.

An example of injecting, for example, solder in which, in terms ofcomposition ratio, tin is at least 90.9 wt %, zinc is 9 wt %, othermetallic element content is less than 0.1 wt % and oxygen content isless than 5 ppm (eutectic point: 198° C.) by using the aforementionedsoldering unit 100 will be described below.

First, solder is thrown into the melting bath 101 and by supplyingnitrogen from the gas supplying unit 109 to the vicinity of the nozzle103 as inactive gas, the solder in the melting bath 101 is held at 208°C. so as to be melted completely. The melted solder is sent to thenozzle 103 and elastic wave output is supplied from the oscillating unit105 to the melted solder S. As a result, fine solder droplets S″ aredischarged from the nozzle 103. The solder droplet S″ reaches apredetermined position on the substrate 115, so that the solder dropletsS are stacked and hardened, thereby gradually increasing the thicknessof solder. With this condition, if the transfer unit 111 is movedhorizontally at a constant speed, spots or a line is formed by solderparticles on the substrate. By supplying elastic wave to the substrate115 by the oscillating unit 125, when the solder droplet S″ comes intocontact with the substrate 115, the oxide film on the surface of thesubstrate 115 is diffused so that the wettability is improved.

By soldering operation using the aforementioned nozzle, soldering inspots of 5 μm to 50 μm in diameter or soldering in line of the samewidth is enabled.

A heating/cooling unit 121 of the transfer unit 111 is used to controlhardening of the solder on the substrate and can be used to increase thethickness of the applied solder. For example, if the temperature of thesubstrate is lowered below the eutectic point of the solder, the solderdroplet reaching the surface of the substrate begins to be hardened. Ifthis substrate is reheated over the eutectic point, the hardened solderbegins to be melted again so that semi-melting condition in which liquidphase and solid phase are mixed is generated. The surface tension ofsolder in this condition is larger than that of the completely meltedsolder. Even if new solder droplet is added to semi-melting conditionsolder, it is not stretched horizontally but integrated with thesemi-melting condition solder so that it is hardened. As a result,solder film having a sufficient thickness is obtained. Thus, a locationand temperature of the heating/cooling unit 121 of the transfer unit 111are determined so that new solder droplet can be added to solder in thesemi-melting condition on the substrate 115. As a result, by stackingsoldering spots by means of a plurality of the nozzles, thick solderingfilm is formed.

A soldering apparatus 200 shown in FIG. 12 comprises a nozzle 203 havinga plurality of nozzle ports 203 a′, 203 b′ and 203 c′. On the insidewalls of the nozzle 203 formed is a metallic oxide film 221 of chromiumoxide. Solder droplets Sa′, Sb′ and Sc′ are discharged from therespective nozzle ports by oscillating units 105 a, 105 b, and 105 c andreach the substrate 115. Parts having the same reference numerals as inFIG. 11 perform the same action as corresponding parts of the solderingunit 100, thus a description thereof is omitted.

The soldering method mentioned in the present invention can be executedby using a nozzle unit in which vibration energy is supplied from thenozzle itself to a base metal to be soldered. For example, the followingnozzle unit is available. An elastic wave oscillator is provided in thevicinity of a nozzle connected to a melting bath in which solder ismelted under non-oxidizing environment and the nozzle is sheathed sothat the nozzle tip is covered with non-oxidizing gas environment andnon-oxidizing gas such as nitrogen is circulated in the sheath. Whilesupplying non-oxidizing gas to the nozzle tip of such a nozzle unit, thenozzle is approached to the base metal to be soldered, and then thenozzle tip is made into contact with the base metal. While making meltedsolder supplied from the melting bath to the nozzle tip in contact withthe base metal, vibration energy is applied to the nozzle. After that,the nozzle is taken away from the base metal while a predeterminedamount of the solder is pushed out from the nozzle tip. With thisoperation, soldering in fine parts can be done. It is permissible tokeep the nozzle tip slightly away from the base metal and supplyvibration energy while only the melted solder is kept into contact withthe base metal. By moving the nozzle while the melted solder is pushedout continuously, linear soldering can be carried out.

FIG. 13 shows an example of the screen printing mechanism using a screenin which metallic oxide film is formed. This screen printing mechanism300 comprises a rotary unit 302 provided with squeegees 301, 301′, ascreen 304 having slits 303, and a base 305 for supporting the substrateB. On inside walls of the slits 303 in the screen 304 formed are zincoxide films 306. The zinc oxide film 306 is formed by applying zincoxide powder diffused in a liquid, on the inside wall of the slits 303,drying it and baking by heating.

The screen 304 of the screen printing mechanism 300 is set at anappropriate position such that it overlies the substrate B, and solderpaste SP is supplied to an end on the screen 304. The rotary unit 302 ismoved horizontally such that the squeegee 301 slides over the screen 304so as to carry solder paste SP to the other end of the screen 304.Consequently, the slits 303 are filled with the solder paste SP.Further, the rotary unit 302 is rotated so as to make the squeegee 301′into contact with the screen 304 and return the squeegee 301′ to thefirst end of the screen 304 thereby completing filling of the solderpaste SP. After that, the solder paste SP in the slit 303 is heated soas to reflow, in order to evaporate or decompose flux contained in thesolder paste SP, thereby removing it. Then the solder components of thesolder paste SP is melted. After that, by cooling, the solder componentis hardened in the slit 303. By raising the screen 304 to remove it fromthe substrate B, only the hardened solder component is left on thesubstrate B. Since the inside wall of the slit 303 has oxide film,solder component is prevented from adhering to the inside wall of theslit 303.

According to the present invention, melted metal containing zinc whichwas conventionally assumed to be inappropriate for soldering work isutilized to bond the members to be bonded, and bonding of electronicparts without lead can be formed. Further, since the bonding portionsformed on the substrate according to the present invention has a surfacezinc layer, migration is prevented in the bonding portions of theelectric/electronic devices which are assembled using this substrate.Therefore, high quality can be assured in high density mountingelectric/electronic devices.

EXAMPLES

Now the present invention will be described in detail with reference toexamples.

Example 1

According to composition shown in Table 2, tin and zinc were mixed andheated so as to be melted. Then, phosphor was added and mixed. Then slagwas removed and the melted material was poured into a mold. Tin-zincsolder alloys shown in Sample Nos. 1 to 8 were obtained. In the samemanner, the tin-zinc solder alloy of the Sample No. 9 was obtained.Table 2 shows a result of measurement of the content of oxygen andmelting point.

Next, a pair of copper plates were soldered under nitrogen environment,using each of the solder alloys of the specimens 1 to 9 to bond thecopper pplates. At this time, the heating temperature for soldering waskept in a temperature range 10 to 30 K higher than the melting point ofeach specimen. During soldering work, wettability of each melted solderalloy was checked by the eyes and estimated by whether at least 98% ofthe surface of the copper plates was wetted by the solder alloy (A) ornot (B). After soldering, ductility of the solder bonding was measuredby the tensile test and estimated by whether ductility reached to 40%(A) or not (B). Table 2 shows a result of this survey.

TABLE 2 Soldering Alloy Cont. Characteristic Sample Composition(wt %)Oxgen M.P. Wetta- duc- No. Zinc Others Tin (ppm) (K) bility safetytility 1 9 — balance 20 471 A A A 2 9 — balance 100 471 A A A 3 9 —balance 150 471 B A A 4 5 — balance 32 486 A A A 5 9 Bi:5 balance 28 464A A B 6 9 In:5 balance 18 452 A A B 7 — Pb:37 balance 121 456 A B A

As evident from Table 2, the tin-zinc alloy solders of the Sample Nos. 1to 4 have a low melting point and as excellent wettability as thetin-lead alloy solder of the Sample No. 9. That is, the solder of SampleNos. 1 to 4 have an excellent soldering characteristic. The letter B ofthe estimation of the safety of tin-lead means that there is a fear intoxicity affecting the environment.

In regard to the alloy solders of Sample Nos. 5 and 6, the results ofductility means that addition of bithmus or indium inhibits formation ofthe surface zinc layer on the bonding interface, which causesdeterioration of ductility. Moreover, it also causes difficulty inseparation and purification of each component of the solder forrecycling use.

Example 2

A solder was prepared under very low oxygen environment using phosphorlike in the example 1, so as to obtain a solder in which tin is at least90.9 wt %, zinc is 9 wt %, oxygen content is at most 5 ppm and otherelement content is less than 0.1 wt %. Then that solder was thrown intoa melting bath 101 of the soldering unit 100 of FIG. 11. The environmentwas kept at 220° C. which is 22° C. higher than the eutectic point of198° C. as its theoretical value to melt the solder completely. Theinside wall of a nozzle 103 of the soldering unit 100 had an chromiumoxide film formed in thickness of 100 Å. A bore of the nozzle port 103′was 1 mm. When the temperature of solder reached 190° C., nitrogen gaswas supplied as inactive gas from a gas supplying unit 109. As a result,oxygen concentration in the vicinity of the nozzle port 103′ became lessthan 250 ppm.

The nozzle port 103 was heated at 220° C. and while supplying meltedsolder from the melting bath 101 to the nozzle 103, elastic wave havinga frequency of 15 to 30 kHz was output from an oscillator 105 providedon the nozzle 103. Then, it was recognized that fine solder particleswere emitted from the nozzle.

Then, using a substrate 115 having a specification described below, thenozzle 103 was positioned so that a distance between the nozzle port103′ and the substrate 115 was 5 mm when the substrate 115 waspositioned just below the nozzle 103. The substrate was movedhorizontally at a speed of 3 cm/second passed through a heated range at210° C. by a heater. Melted solder drops were emitted from the nozzleport 103′ when the substrate passed a predetermined position, so thatthey dropped on an objective pad position on the substrate. By movingthe substrate in the pad direction, emission of solder drops wascontinued to apply the solder onto the entire surface of the pad with apredetermined length. The applied solder began to harden when reachingto a temperature below the eutectic point.

[Specification of the substrate]

Size: 35 mm×100 mm

material: conductive portion/copper, nickel plating and gold flashplating resin portion/glass epoxy resin, resist processing pad pattern:150 μm in width×15 mm in length interval between pads: 100 μm

After cooling, a portion in which solder was hardened was slicedvertically and its cross section was observed. Then, it was confirmedthat the wetting angle was less than 90 degrees, indicating thatwettability with solder was excellent. There was no bridge formedbetween pads. Further, an analysis in depth direction was conducted inthe center portion of the pad up to a depth of about 100 Å according toAuger electron spectroscopy. As a result, a small amount of carbonattached to the surface was found and a layer containing zinc and oxygenin thickness of about 70 Å was found on the surface. However, no tin wasfound. Further therebelow, a layer in which zinc and tin existed inweight ratio of about 1:9 was found. The carbon on the surface isconsidered to have been generated due to carbonization of substance inthe neighborhood such as resist. It is considered that a layercontaining a large amount of zinc and oxygen existing on the surfaceinhibits penetration of oxygen from the atmosphere into the insidethereof.

Using the above substrate, an assembly was formed by mounting QFP asactive part, and a resistor and a display unit as passive part, and thenoperated. Consequently, generation of migration was not found.

A substrate in which the pad width was 50 μm and pad interval was 50 μmwas coated with the solder in the same manner as above. Then,wettability of the solder and pad was observed after cooling. Then,wetting angle was less than 90 degrees and it was confirmed that thewettability with the solder was excellent. Further, there was no bridgewith the solder found between the pads.

Example 3

Alloy particles of 10 to 50 μm in diameter, having the same compositionas the solder used in Example 2 was prepared. A substrate according to aspecification below was moved at a speed of 3 cm/second so that itpassed through an exit of a passage filled with argon gas heated at 220°C. Alloy particles were introduced to this passage and carried to reachthe substrate. During passing through the passage, the surfaces of thealloy particles were melted so that they adhered to the substrate pad.

[Substrate specification]

size: 35 mm×100 mm

material: conductive portion/copper, nickel plating resin portion/glassepoxy resin, resist processing

pad pattern: 200 μm (width)×15 mm (length)

pad interval: 200 μm

After cooling, a portion in which τηε solder was hardened was slicedvertically and its cross section was observed. Then, it was confirmedthat the wetting angle was less than 90 degrees, indicating thatwettability with solder was excellent. There was no bridge formedbetween pads. Further, an analysis in depth direction was conducted inthe center portion of the pad up to a depth of about 100 Å according toAuger electron spectroscopy. As a result, a small amount of carbonattached to the surface was found and a layer containing zinc and oxygenin thickness of about 80 Å was found on the surface. Moreover, about 1%tin was detected in this layer. Therebelow, a layer in which zinc andtin existed in weight ratio of about 1:9 was found. On an interfacebetween the pad and the solder, a layer of about 1 μm in thicknesscontaining oxygen of about 10 ppm, in which a ratio of content of zincwas at least 70 wt % was found in accordance with energy diffusion typeX-ray spectral analysis.

Using the above substrate, as shown in FIG. 2, an assembly was formedby-mounting QFP as active part, and a resistor and a display unit aspassive part, and then operated. Consequently, generation of migrationwas not found.

Example 4

Using phosphor like in example 1, a solder in which, in terms ofcomposition ratio, tin was at least 92.4 wt%, zinc was 7 wt %, silverwas 0.5 wt %, other metallic elements content was less than 0.1 wt % andoxygen content was at most 5 ppm was prepared under very low oxygenenvironment. This was processed to foil having an equal thickness of 20μm and wound up in the form of roll.

A PC board having a specification below was introduced to a chip mounterusing a transfer apparatus and the board was stopped at a predeterminedposition by means of a CCD camera. Helium was supplied as inactive gasand while keeping the concentration of oxygen in a range of 50 ppm to250 ppm, the aforementioned solder foil of 20 μm in thickness was placedso as to cover the pad of the substrate and then a metal head (havingchromium oxide film on the surface) was pressed momentarily from abovethe pad at the aforementioned predetermined position so as to apply apressure of 20 MPa. The solder foil was fused to the pad due to theapplied pressure. The solder foil which did not overlap the pad wascollected by winding up. The collected solder foil could be reused as anequal thickness foil.

[Specification of the substrate]

size: 35 mm×100 mm

material: conductive portion/copper resin portion/glass epoxy resin,resist processing

pad pattern: 200 μm in width×15 mm in length

pad interval: 200 μm

Observing a substrate picked out of the mounter by means of amicroscope, no thin spot, drooping, and bridge were seen on the solderadhering to the pad and the solder was spread almost equally at athickness of about 8 μm. Further, analysis in depth direction accordingto Auger electron spectroscopy method was carried out in the vicinity ofthe pad, up to about 120 Å. As a result, a small amount of carbonadheres to the surface and a layer composed of zinc and oxygen of about80 Å in thickness was found on the surface. Further therebelow, a layercontaining zinc and tin in the weight ratio of 1:9 was found. As aresult of measurement in the vicinity of an interface between the padand the solder according to energy diffusion type X-ray spectralanalysis, a layer of about 1 μm in thickness containing oxygen of about10 ppm, in which the ratio of content of zinc was about 80 wt %, wasfound.

Example 5

Using phosphor like in example 1, solder in which, in terms ofcomposition ratio, tin was at least 90.9 wt %, zinc was 9 wt %, othermetallic elements content was less than 0.1 wt % and oxygen content wasat most 20 ppm was prepared under very low oxygen environment. This wasprocessed to foil having an equal thickness of 15 μm and wound up in theform of roll.

Using a substrate having specifications described below and theaforementioned foil, the same operation as in the Example 4 was repeatedso as to obtain a substrate in which the solder foil was fused to thepad.

[Specification of the substrate]

size: 35 mm×100 mm

material: conductive portion/stainless steel SUS 316 resin portion/glassepoxy resin, resist processing

pad pattern: 200 μm in width×15 mm in length

pad interval: 200 μm

Observing the substrate picked out of the mounter by means of amicroscope, no thin spot, drooping or bridge was found on the solderadhering to the pad, and the solder was stretched almost equally.Analysis in the depth direction according to Auger electron spectroscopymethod was carried out up to a depth of about 100 Å in the vicinity ofthe center of the pad. As a result, a small amount of carbon adhere tothe surface, and a layer composed of zinc and oxygen having a thicknessof about 60 Å existed on the surface portion. Further therebelow, alayer in which zinc and tin existed in the weight ratio of 1:9 wasfound.

Example 6

Using phosphor like in example 1, solder in which, in terms ofcomposition ratio, tin was at least 90.9 wt %, zinc was 9 wt %, othermetallic elements content was less than 0.1 wt % and oxygen content wasat most 5 ppm was prepared under very low oxygen environment. This wasprocessed to foil having an equal thickness of 15 μm and wound up in theform of roll.

An IC chip having a specification below was introduced to a chip mounterusing a transfer apparatus and the IC chip was stopped at apredetermined position by means of a CCD camera. Argon gas was suppliedto the IC chip as inactive gas and while keeping the concentration ofoxygen in a range of 50 ppm to 250 ppm, the aforementioned solder foilof 15 μm in thickness was placed so as to cover the lead of the IC chipand YAG laser was irradiated over the lead at a predetermined positionfor 50 m seconds. Due to irradiation of laser, the solder foil was fusedto the pad. The solder foil which did not overlap the lead was collectedby winding up. The collected solder foil could be reused as an equalthickness foil.

[Specification of the IC chip]

material: conductive portion/42 alloy resin portion/unsaturatedpolyester resin

lead pattern: 190 μm in width×14 mm in length

pad interval: 220 μm

Example 7

Using phosphor in the same manner as the example 1, a solder in which,in terms of composition ratio, tin was at least 90.9 wt %, zinc was 8.7wt %, silver was 0.3 wt %, other metallic elements content was less than0.1% and oxygen content was at most 15 ppm was prepared. Then, it wasprocessed to solder powder having an almost equal sphere of 20 μm indiameter. By diffusing 90 parts by weight of this solder powder in 10parts by weight of organic flux (containing 2-alkyl-1,3-hexandiol as amain solvent component, product by Senju Kinzoku Kogyo Kabushiki Kaisyaof Tokyo, Japan), solder paste was prepared.

By using a screen printing machine 300 shown in FIG. 13, a substrate Bhaving specifications below was fixed and purged with nitrogen gas.Then, a screen 304 having specifications below was placed so as tocoincide with the pad of the substrate B, and the solder paste preparedpreviously was placed on one end of the screen 304. By reciprocating arotary body 302 horizontally along the screen 304, the solder paste wascharged in the slit 303 by means of squeegees 301, 301′.

[Specifications of the substrate]

size: 150 mm×150 mm

material: conductive portion/copper resin portion/glass epoxy resin, thesurface processed by resist.

pad pattern: 100 μm

pad interval: 100 μm

[Printing condition]

Printing time: 40 seconds/tact

[Specifications of the screen]

thickness: 0.15 mm

material: stainless steel SUS 316

slit: 100 3m in width×15 mm in length, inside surface coated with zincoxide film

After that, solder paste in the slit 303 was heated at 150° C. so as toreflow, thereby removing flux by evaporation or decomposition. Then, thetemperature was raised to 220° C. to melt the solder powder. After that,the substrate was cooled to harden the solder and the screen 304 wasremoved from the substrate.

Then, the substrate was introduced to the chip mounter by means of thetransfer apparatus, and the substrate was stopped at a predeterminedposition by means of a CCD camera. Helium was supplied to the substrateas inactive gas, and while keeping the concentration of oxygen in arange of 50 ppm to 250 ppm, a QFP chip was mounted on the soldered pad.Next, the substrate in which the chip was mounted was introduced intothe reflow furnace. Nitrogen gas was supplied to the reflow furnace asinactive gas and while preventing oxygen to exist inside, thetemperature was raised up to 220° C. so as to melt the solder. Afterthat, the substrate was picked out of the furnace and left to cool.

Observing the picked out substrate by means of a microscope, no droopingof solder or formation of bridge was found. Further, bonding strengthwas measured with respect to the bonded chip. As a result, the bondingstrength was 8 N on average, and it was found that it indicated almostthe same or more value as indicated by using a conventional tin-leadseries paste solder.

Example 8

Using phosphor in the same manner as in the example 1, solder in which,in terms of composition ratio, tin was at least 90.9 wt %, zinc was 9 wt%, other metallic element content was less than 0.1% and oxygen contentwas at most 15 ppm was prepared and it was processed to solder powderhaving an almost equal sphere of 10 μm in diameter. Then 88 parts byweight of this solder powder was diffused into 12 parts by weight oforganic series flux (containing 2-alkyl-1,3-hexandiol as a main solventcomponent, product by Senju Kinzoku Kogyo Kabushiki Kaisya of Tokyo,Japan) so as to prepare solder paste.

Using the aforementioned solder paste and a screen having specificationsbelow as a screen of the screen printing machine 300, the same operationas in the Example 7 was repeated to mount the small size active part ona silicone wafer having specifications below.

[Specification of the silicone wafer]

size per section: 15 mm×15 mm

surface: Polishing

Bonding pattern: 100 μm

Bonding interval: 200μm

[Printing condition]

time: 40 seconds/tact

[Screen specifications]

thickness: 0.15 mm

material: stainless steel SUS316

Slit: 100 μm, inside surface coated with titanium oxide film.

Observing a picked out silicone wafer by means of a microscope, nodrooping of solder or formation of bridge was found. The bondingstrength was measured about the bonded part. As a result, the bondingstrength was 14 N on average, and it was found that almost the same ormore strength as a conventional conductive adhesive agent was indicated.

Example 9

Using phosphor in the same manner as in the example 1, solder in which,in terms of composition ratio, tin was at least 90.9 wt %, zinc was 9 wt%, oxygen content was at most 5 ppm, and other metallic element content(Bi, Ag, In, Cu) was less than 0.1 wt % was prepared under very lowoxygen environment.

This solder was thrown into a melting bath of argon environment, and bykeeping the temperature at 220° C. which is 22° C. higher than theeutectic point of 198° C. as a theoretical value, the solder was melted.When the solder temperature reached 190° C., argon gas was supplied tothe vicinity of the nozzle port which was connected to the melting bathas inactive gas. As a result, the concentration of oxygen in thevicinity of the nozzle became less than 250 ppm.

Next, a copper pad on which a fine device could be mounted and aninsulating substrate for high density mounting having copper wiring onthe surface, based on specifications below were moved horizontally at aspeed of 3 cm/second so as to pass Just below the nozzle. The meltedsolder was dropped from the nozzle and applied onto the wiring and pad.When the temperature of the substrate dropped below the eutectic pointtemperature, the coated solder was immediately hardened. Then, thesubstrate was turned back and the same operation was repeated so as toapply the solder onto wiring on the back face.

As shown in FIG. 3, a packaged LSI as an active part and a resistor as apassive part were mounted by solder on the front surface of a substratecoated with the solder. Further, a resistor was bonded to a pad on theback face of the substrate and fixed securely.

[Specification of the substrate]

size: 35 mm×100 mm

material: conductive portion/copper, nickel plating or gold flashplating resin portion/glass epoxy resin, resist treatment

pad pattern: 100 μm in width×15 mm in length

pad interval: 200 μm

After cooling, a portion in which solder was hardened was slicedvertically and observed in its cross section. As a result, it was foundthat wetting angle of the solder relative to the pad and wiring was lessthan 90 degrees, indicating that the wettability to the solder wasexcellent. No bridge was seen between the pads. Linear analysis ondistribution condition of zinc in the vicinity of an interface betweenthe pad and solder was carried out in the center of the pad. FIG. 14shows this result. Referring to FIG. 14, the region α indicates a padand the region β indicates solder. From a result of the linear analysis,it is found that the layer L of 1 μm containing a large amount of zinccovers the pad. The concentration of zinc rises steeply on an interfaceof the pad in the layer L and then gradually decreases as it leaves theinterface toward inside of the solder. As described above, the layer Lwas formed such that zinc was concentrated from the solder on thebonding interface. The layer L acts to enhance bonding between the padand the solder. Further, the layer L contains oxygen of about 1 to 30ppm, which is larger than the amount of oxygen in the solder in theregion β. That is, oxygen in the solder is concentrated upon the layerL. It is considered that this also contributes to enhancement of thebonding of the solder.

Using tin-zinc solders having different amounts of zinc, according tothe same operation above, a copper pad, a silver pad and a gold pad ofthe insulating substrate were soldered so as to mount electronic parts.The ratio of content of zinc in the layer L, composed of mainly zinc,formed on an interface of the soldered bonding portion was measured andfurther a tension force necessary for peeling out the mounted part fromthe soldered bonding portion was measured so as to estimate the bondingstrength. Table 3 shows the result of this estimation. Referring toTable 3, the letter A indicates the bonding showing the same as orhigher bonding strength than that of ordinary tin-lead solders (about3.5 kgf/mm²), and the letter B indicates parts inferior to the tin-leadsolder or which failed to be bonded.

TABLE 3 Bonding Strength Zinc Content in the Layer L (wt %) Pad 50 8899.5 copper B A A silver B A A gold B A A

As evident from the above description, bonding strength of the solderdiffers depending on the ratio of content of zinc in the layer L.Bonding portions which indicated excellent bonding have fewer finecavities created in the solder than ordinary tin-lead solders. It isconsidered that this improved the tension strength.

From Table 3, it is found that the layer L composed of mainly zinc formsexcellent bonding with gold, silver and copper. Further, even if amember to be bonded is a mixture material composed of mainly gold,silver or copper and other metal, the same excellent bonding can beachieved.

Table 4 shows a relation between the compositions of the layer L and thehardened tin-zinc solder and bonding strength in a case when the bondingis formed in such a condition that the layer L composed of mainly zinc,exists between a copper member to be bonded and the tin-zinc solder.Control of the compositions of the layer and the hardened tin-zincsolder was achieved by changing the composition of the used tin-zincsolder containing 5 to 13 wt % zinc, changing the temperature at whichthe melted tin-zinc solder was held in the melting bath within a rangeof 205 to 235° C., and changing the cooling speed of the solder appliedto the copper member. In Table 3, the letter A shows the bondingstrength being 5 kgf/mm² 1548 or more, the letter B shows that 3.5 to5.05 kgf/mm², and the letter C shows that of being less than 3.5 5kgf/mm².

TABLE 4 Bonding Strength Zinc Content Zinc Content in the Layer L (wt %)of Hard Solder 50 88 95 99.5 4.8 C C C C 5.8 C B B B 8.8 C B A A 10 C BA A 11.7 C B B B 12.3 C C C C

From the above result, it is understood that when the ratio of contentof zinc in the layer L is 88 to 99.5 wt %, and when the hardened soldercontains 5.8 to 11.7 wt % zinc, an excellent bonding strength can beobtained.

Example 10

Using phosphor in the same manner as in the example 1, solder in which,in terms of composition ratio, tin was at least 90.9 wt %, zinc was 9 wt%, oxygen content was at most 5 ppm and other metallic elements contentwas less than 0.1 wt % was prepared under very low oxygen environment.Argon gas was blown into a leveler plating solder bath so as to obtainnon-oxidizing environment, in which solder is thrown and melted. Asubstrate having specifications below, provided with the pad is immersedin melted solder. Consequently, a substrate in which leveler platinglayer was uniformly formed by solder was obtained.

[Specification of the substrate]

size: 35 mm×100 mm

material: conductive portion/iron resin portion/glass epoxy resin,resist treatment

pad pattern: 200 μm in width×15 mm in length

pad interval: 200 μm

Observing an obtained substrate by means of a microscope, it was foundthat the wetting angle between the solder and the pad was less than 90degrees, indicating that the wettability to solder was excellent. Nobridge due to solder was seen between the pads. Further, linear analysisaccording to energy diffusion type X-ray spectral method was carried outin the vicinity of an interface between the solder and the pad, in thecenter of the pad. Consequently, in a solder layer of 12 μm inthickness, a layer having a thickness of about 2 μm, in which thecontent of zinc was about 75 wt % was found.

Example 11

Using an IC chip having specifications below instead of the substrate,parts mounting was carried out as shown in FIG. 2 and the same operationas in the Example 9 was repeated so as to obtain an IC chip on whichparts were mounted. QFP as an active part and resistors as a passivepart were mounted on the back face of the IC chip and LSI was mounted asan active part on the back face thereof. Mounting parts on both sidescan raise the mounting density.

[Specification of the IC chip]

material: conductive portion/42 alloy resin portion/unsaturatedpolyester resin

Lead pattern: 190 μm in width×14 mm in length

Pad interval: 220 μm

As described above, according to the present invention, by using abonding material composed of metallic composition without lead,electronic parts are mounted on a mounting substrate. As a result, anelectronic-part-mounted substrate in which migration is prevented isobtained. Further, according to the present invention, the bondingmaterial can be supplied to fine portions accurately, so that mountingof parts on the high density mounting substrate is facilitated. Thebonding strength is excellent. Further, according to the presentinvention, there is provided a mounting substrate having bonded partslikely to have a sufficient strength at bonding portions formed withbonding material, and a sufficient resistance to distortion. Thus, thisis advantageous in terms of industry and environmental protection.

It must be understood that the invention is in no way limited to theabove embodiments and that many changes may be brought about thereinwithout departing from the scope of the invention as defined by theappended claims.

What is claimed is:
 1. A method of bonding an electronic part to asubstrate with a solder material, comprising: disposing a soldermaterial between the electronic part and the substrate, the soldermaterial essentially consisting of a zinc component and a tin componentwherein the content of an oxygen component of the solder material isreduced to 100 ppm or less; melting at least the surface of the soldermaterial to wet the electronic part and the substrate with the meltedsolder material in a substantially non-oxidizing atmosphere; andhardening the melted solder material, thereby bonding the electronicpart to the substrate.
 2. The bonding method according to claim 1,wherein said zinc component is present from about 3% to about 12% of thesolder composition.
 3. The bonding method according to claim 1, whereinsaid solder composition includes less than 0.1% by weight of othermetallic components.
 4. The bonding method according to claim 1, whereinthe substantially non-oxidizing atmosphere comprises argon gas ornitrogen gas, and the solder material at the melting step is heated to atemperature which is 10 to 30° C. higher than the melting temperature ofthe solder material.
 5. The bonding method according to claim 1, furthercomprising: providing, at the melting step, oscillatory wave energy toimprove wetting of the electronic part or the substrate with the meltedsolder material.
 6. The bonding method according to claim 1, furthercomprising, before the disposing step: preparing a molten metal mixturecontaining the zinc component and the tin component such that the ratioof the zinc component to the tin component is substantially the same asthe solder material; and treating the molten metal mixture with aneffective amount of an oxygen scavenger for reducing the content ofoxygen component to 100 ppm or less to remove the oxygen component asslag, whereby said solder material is provided from the molten metalmixture, wherein the oxygen scavenger comprises an element selected fromthe group consisting of phosphor and magnesium.
 7. The bonding methodaccording to claim 1 wherein the amount of the oxygen scavenger is 0.01to 0.1% by weight to the molten metal mixture.
 8. A method of bonding anelectronic part to a substrate with a solder material, comprising:disposing a solder material between the electronic part and thesubstrate, the solder material comprising 3 to 12% by weight of a zinccomponent and a tin component wherein the content of an oxygen componentof the solder material is reduced to 100 ppm or less; melting at leastthe surface of the solder material to wet the electronic part and thesubstrate with the melted solder material in a substantiallynon-oxidizing atmosphere; and hardening the melted solder material,thereby bonding the electronic part to the substrate.
 9. The bondingmethod according to claim 8, wherein the solder material comprisessubstantially no bisthmus or no indium.
 10. The bonding method accordingto claim 9, wherein the solder material is substantially a binary alloyessentially consisting of the zinc component and the tin component suchthat the content of other metallic components except for the zinccomponent and the tin component is reduced to less than 0.1% by weight.11. The bonding method according to claim 8, wherein the solder materialfurther comprises a metal component which is selected from the groupconsisting of antimony, indium, gold, silver and copper, at a content of3% by weight or less.
 12. The bonding method according to claim 8,wherein the substantially non-oxidizing atmosphere comprises argon gasor nitrogen gas, and the solder material at the melting step is heatedto a temperature which is 10 to 30° C. higher than the meltingtemperature of the solder material.
 13. The bonding method according toclaim 8, further comprising: providing, at the melting step, oscillatoryenergy to improve wetting of the electronic part or the substrate withthe melted solder material.
 14. The bonding method according to claim 8,further comprising, before the disposing step: preparing a molten metalmixture having substantially the same composition as the soldermaterial; and treating the molten metal mixture with an effective amountof an oxygen scavenger for reducing the content of oxygen component to100 ppm or less to remove the oxygen component as slag, whereby saidsolder material is provided from the molten metal mixture, wherein theoxygen scavenger comprises an element selected from the group consistingof phosphor and magnesium.
 15. The bonding method according to claim 14,wherein the amount of the oxygen scavenger is 0.01 to 0.1% by weight tothe molten metal mixture.